A membrane system was further developed that exhibited very high transport selectivity for anions while at the same time showing electrical selectivity for cations. The system consisted of a liquid anion exchanging membrane portion and a porous solid cation exchanging portion. As an example, when this system was interposed between .001 N Rb SCN and .1 N Rb SCN solutions, with a RB*SC*N radioactive tracer in the more dilute solution, the exchange of SCN- across the membrane was 3,680 times faster than the exchange of Rb ion. However, as calculated from the membrane potential, the electrical transference number of the Rb ion was .943 while that of the SCN- was .057 or in other words, the dissociated rubidium ions were traversing the membrane 16 times faster than the thiocyanate ions. In the latter case, the potential is cause only by the ions that are dissociated while going through the membrane, primarily through the porous portion. In the former case both the dissociated and undissociated ions (going primarily through the liquid portion of membrane) are counted. The ions that go through the membrane in an undissociated state far out number those going through in the dissociated state. The large transport selectivity was accomplished by taking into account how the Donnan distribution would affect the radioactive tracer.